I'm trying to make a C++ class using the Code::Blocks IDE and there is a field called "Guard block." I've done a search and haven't been able to find any useful information. What is this field for? Thanks.
Guard blocks are used to protect against the inclusion of a header file multiple times by the same compilation unit (c++ file). They look something like this:
// Foo.h
#ifndef INCLUDE_FILE_NAME_HERE_H_
#define INCLUDE_FILE_NAME_HERE_H_
class Foo
{
};
#endif
If you include the same file multiple files, you will end up with multiple definition error. Using of include guards isn't necessary in small projects but becomes critical in any medium to large sized projects. I use it routinely on any header files I write.
Guard blocks are used to prevent a header file being included multiple times in a single translation unit. This is often a problem when you include a number of header files that in turn include common standard header files.
The problem with multiple inclusions of the same file is that it results in the same symbol being defined multiple times.
Guard clauses can be handled with #define and #ifdef statements but are much simpler with the non-standard, but universal, #pragma once.
// foo.h
#pragma once
int foo(void);
// etc.
Related
What happens if I include iostream or any other header file twice in my file?
I know the compiler does not throw error.
Will the code gets added twice or what happens internally?
What actually happens when we include a header file ?
Include guard prevents the content of the file from being actually seen twice by the compiler.
Include guard is basically a set of preprocessor's conditional directives at the beginning and end of a header file:
#ifndef SOME_STRING_H
#define SOME_STRING_H
//...
#endif
Now if you include the file twice then first time round macro SOME_STRING_H is not defined and hence the contents of the file is processed and seen by the compiler. However, since the first thing after #ifdef is #define, SOME_STRING_H is defined and the next time round the header file's content is not seen by the compiler.
To avoid collisions the name of the macro used in the include guard is made dependent on the name of the header file.
Header files are simple beasts. When you #include <header> all that happens is that the contents of header basically get copy-pasted into the file. To stop headers from being included multiple times, include guards are used, which is why in most header files you'll see something akin to
#ifndef SOME_HEADER_FILE_GUARD
#define SOME_HEADER_FILE_GUARD
//Contents of Header
#endif
It simply gets skipped over, due to preprocessor code along the following lines:
#ifndef MY_HEADER_H
#define MY_HEADER_H
<actual header code here>
#endif
So if you include twice, then MY_HEADER_H is already defined and everything between the #ifndef and #endif is skipped by the preprocessor.
It depends. With the exception of <assert>, the standard requires the
second (and later) includes of a standard header to be a no-op. This is
a characteristic of the header, however; the compiler will (at least
conceptually) read and include all of the header text each time it
encounters the include.
The standard practice for avoiding multiple definitions in such cases is
to use include guards: all of the C++ code in the header will be
enclosed in something like:
#ifndef SPECIAL_NAME
#define SPECIAL_NAME
// All of the C++ code here
#endif SPECIAL_NAME
Obviously, each header needs a different name. Within an application,
you can generally establish conventions based on the filename and
location; something like subsystem_filename, with
characters not legal in a C++ symbol (if you're using them in your
filenames) mapped (and very often everything upper cased). For
libraries, the best practice would be to generate a reasonably long
random sequence of characters; far more frequent (although certainly
inferior from a quality of implementation standpoint) is to ensure that
every such symbol begin with a documented prefix.
A system library can, of course, use reserved symbols (e.g. a symbol
starting with an underscore followed by a capital letter) here, to
guarantee that there is no conflict. Or it can use some entirely
different, implementation dependent technique. Microsoft, for example,
uses a compiler extension #pragma once; g++ uses include guards which
always start with _GLIBCXX (which isn't a legal symbol in user code).
These options aren't necessarily available to you.
When you include a header file, all its contents get copied to the line where the "#include" directive was placed. This is done by the preprocessor and is a step in the compilation process.
This process is the same for standard files like iostream or user-made ".h" files stored in local directory. However, the syntax slightly differs.
We use #include <filename> for files like 'iostream' which are stored in the library. Whereas, for header files in the local directory, we use #include "filename.h".
Now, what if we include header files twice:
Ideally speaking the content should be copied twice. But...
Many header files use the modern practice of mentioning #pragma once which instructs the pre-processor to copy contents only once, no matter how many times the header file is included.
Some very old codes use a concept called 'include gaurds'. I won't explain it as the other answers do so very well.
Using pragma once is the easy and the modern approach, however, include guards were used a lot previously and is a relatively complicated fix to this issue.
I don't wish to create a wall of code so if you need any code just ask and i will post. I am getting multiple definition errors and i have no idea why. i believe the error is occurring because i am attempting to declare a variable in one header file(as well as create an object for it in the same header) and then use it among 4 different files.and for each file i use the header in i get multiple definition, with the small amount of info i have given you any ideas as to why this is occurring?
please ask if more information is required to solve the question.
I am using Code::Blocks ide mingW compiler and programming on Windows 7
EDIT
Thanks you all for your extremely fast help you have solved my problem :D
Don't declare it in a header file (although technically, I mean don't define it there).
In your header file you should have:
extern int variableIWantOneCopyOfOnly;
and then, in a single C source file that you will link in, put:
int variableIWantOneCopyOfOnly;
The former will let all your source files know about the existence of the variable but without allocating space for it. The single translation unit containing the second line will allocate the space for it and the linker will fix up all the references to it.
Include guards won't help with multiple definitions since they only stop the inclusion within a single translation unit.
By that I mean, if you have the following files:
xyz.h: xyz1.c: xyz2.c:
#ifnef XYZ #include "xyz.h" #include "xyz.h"
#define XYZ
int a;
#endif
and you execute a command like:
gcc xyz1.c xyz2.c
then they will both get the own copy of a, not share a copy, because the scope of the include guard is the translation unit, not the executable created by the linker.
Have you used include guards in your header? Generally, you want to structure your headers something like this:
#ifndef _HEADER_FILE_H
#define _HEADER_FILE_H
// actual definitions
#endif
There's also #pragma once, but that is not standard.
"as well as create an object for it in the same header"
If you define things in your headers, and then try to link objects from multiple translation units including that header, you'll get multiple definition errors.
If the header's called say x.h, create an x.cc (or x.c++, x.cpp or whatever you normally do) that includes the header. Move the definition of the object in there. Compile it and link it with the other objects in your application.
the multiple definition error means you are declaring many times the same variable. Regarding what you are doing, this occurs because you are including 4 times the same file (so you are declaring 4 times your variables.
Add the preprocessor
#ifndef _HEADERFILE_H
#define _HEADERFILE_H
// Your declarations
#endif
or if you are using Visual studio just add this on the first line of the file:
#pragma once
You should move the actual definition into a c++ (.cc .cpp whatever) file and only leave the declaration within the header. Also you will need to prefix the declaration with 'extern'.
In C++ what is the purpose of header guard in C++ program.
From net i found that is for preventing including files again and again but how do header guard guarantee this.
The guard header (or more conventionally "include guard") is to prevent problems if header file is included more than once; e.g.
#ifndef MARKER
#define MARKER
// declarations
#endif
The first time this file is #include-ed, the MARKER preprocessor symbol will be undefined, so the preprocessor will define the symbol, and the following declarations will included in the source code seen by the compiler. On subsequent #include's, the MARKER symbol will be defined, and hence everything within the #ifnde / #endif will be removed by the preprocessor.
For this to work properly, the MARKER symbol needs to be different for each header file that might possibly be #include-ed.
The reason this kind of thing is necessary is that it is illegal in C / C++ to define a type or function with the same name more than once in a compilation unit. The guard allows you to #include a header file without worrying if has already been included. Without the guard, multiple inclusions of the same header file would lead to unwanted redeclarations and compilation errors. This is particularly helpful when header files need to #include other header files.
In short, it doesn't prevent you from #include-ing a file again and again. Rather, it allows you to do this without causing compilation errors.
The purpose of header guards is to prevent issues where some code may appear only once per translation unit.
One example is a struct. You cannot redefine a struct even if the second definition is identical. So, if you try to compile the following:
struct foo { int x; };
struct foo { int x; };
The compiler will fail because of the redefinition.
It can be hard to guarantee you only include a header one time (this happens when headers include other headers). If your header has struct definition, this will cause the compile to fail. Header guards are the easy trick so that even if a header is included multiple times, it's contents only appear a single time.
In eclipse, whenever I create a new C++ class, or C header file, I get the following type of structure. Say I create header file example.h, I get this:
/*Comments*/
#ifndef EXAMPLE_H_
#define EXAMPLE_H_
/* Place to put all of my definitions etc. */
#endif
I think ifndef is saying that if EXAMPLE_H_ isn't defined, define it, which may be useful depending on what tool you are using to compile and link your project. However, I have two questions:
Is this fairly common? I don't see it too often. And is it a good idea to use that rubric, or should you just jump right into defining your code.
What is EXAMPLE_H_ exactly? Why not example.h, or just example? Is there anything special about that, or could is just be an artifact of how eclipse prefers to auto-build projects?
This is a common construct. The intent is to include the contents of the header file in the translation unit only once, even if the physical header file is included more than once. This can happen, for example, if you include the header directly in your source file, and it's also indirectly included via another header.
Putting the #ifndef wrapper around the contents means the compiler only parses the header's contents once, and avoids redefinition errors.
Some compilers allow "#pragma once" to do the same thing, but the #ifndef construct works everywhere.
This is just a common way to protect your includes - in this way it prevents the code from being included twice. And the identifier used could be anything, it's just convention to do it the way described.
Is it common? Yes - all C and C++ header files should be structured like this. EXAMPLE_H is a header guard, it prevents the code in the header being included more than once in the same translation unit, which would result in multiple definition errors. The name EXAPMLE_H is chosen to match the name of the header file it is guarding - it needs to be unique in your project and maybe globally as well. To try to ensure this, it's normal to prefix or suffix it with your project name:
#define MYPROJ_EXAMPLE_H
for example, if your project is called "myproj". Don't be tempted into thinking that prefixing with underscores will magically make it unique, by the way - names like _EXAMPLE_H_ and __EXAMPLE_H__ are illegal as they are reserved for the language implementation.
Always do this at the top of a header file. It's typically called a header guard or an include guard.
What it does is make it so that if a header file would be included multiple times, it will only be included once. If you don't do it, then you'll end up with errors about things being defined multiple times and things like that.
The exact define doesn't matter that much, though typically it's some variation on the file name. Basically, you're checking whether the given macro has been defined. If it hasn't, then define it, and continue with including the file. If it has, then you must have included the file previously, and the rest of the file is ignored.
This is an include guard. It guarantees that a header is included no more than once.
For example, if you were to:
#include "example.h"
#include "example.h"
The first time the header is included, EXAMPLE_H_ would not be defined and the if-block would be entered. EXAMPLE_H_ is then defined by the #define directive, and the contents of the header are evaluated.
The second time the header is included, EXAMPLE_H_ is already defined, so the if-block is not re-entered.
This is essential to help ensure that you do not violate the one definition rule. If you define a class in a header that didn't have include guards and included that header twice, you would get compilation errors due to violating the one definition rule (the class would be defined twice).
While the example above is trivial and you can easily see that you include example.h twice, frequently headers include other headers and it's not so obvious.
Consider this
File foo.c:
#include foo.h
#include bar.h
File bar.h
#include <iostream>
#include foo.h
Now, when we compile foo.c, we have foo.h in there twice! We definitely don't want this, because all the functions will throw compile errors the second time around.
To prevent this, we put the INCLUDE GUARD at the top. That way, if it's already been included, we define a preprocessor variable to tell us not to include it again.
It's very common (often mandated), and very frustrating if someone doesn't put one in there. You should be able to simply expect that each .h file has a header guard when you included. Of course, you know what they say when you assume things ("makes an ass of u and me") but that should be something you're expecting to see.
This is called an include guard and is indeed a common idiom for C/C++ header files. This allows the header file to be included multiple times without multiply including its contents.
The name EXAMPLE_H_ is an arbitrary convention but has to obey naming rules for C preprocessor macros, which excludes names like example.h. Since C macros are all defined in a single global namespace, it is important that you do not have different header files that use the same name for their include guard. Therefore, it is usually a good idea to include the name of your project or library in the include guard name:
#ifndef __MYPROJECT_EXAMPLE_H__
...
Why does each .h file starts with #ifndef #define #endif? We can certainly compile the program without those directives.
It's a so-called "include guard". The purpose is to prevent the file from having to be parsed multiple times if it is included multiple times.
It prevents multiple inclusions of a single file. The same can be done using
#pragma once
directive, but those #ifndefs are standard thus supported by every compiler.
It is called an include guard. You can write without them until you start writing large programs and find out that you need to include the same .h file more than once, directly or indirectly, from a .c file. Then without include guards you would get multiple definition errors, but with them, the header file contents are parsed only once and skipped all the subsequent times, avoiding those errors. It`s a good practice to always use them.
If I understand correctly, you want to know if, in the absence of include guards, can including a header file multiple times cause an error or dangerous behavior. This is after excluding multiple definitions, etc.
Imagine a malicious programmer, whose header file doesn't have the include guards. His header file defines one macro, SZ, which is a size that you use for your statically allocated arrays. The programmer could write his header file like this:
#ifndef SZ
#define SZ 1024
#else
#if SZ == 1024
#undef SZ
#define SZ 128
#else
#error "You can include me no more than two times!"
#endif
#endif
Now, if you include the header file once, you get SZ equal to 1024. If you include it twice, SZ becomes 128. Of course, most real-world programmers are not malicious, and no one really writes code like above.
Note that the C standard allows assert.h to be #included more than once with different behavior depending upon whether NDEBUG is defined at the time of inclusion of assert.h. So, assert.h can't have include guards. That is a feature, not a bug, though.
If a header file contains definition likeint i; than, being included several times without a guard, will produce a compilation error.ifndef checks that some preprocessor variable is not defined (and it is not, for a first time), then defines it explicitly to avoid being captured again.
In MSVC you can also use #pragma once instead of ifndef's.